Code Division Multiple Access (CDMA) is a modulation and multiple access scheme that uses wideband spread spectrum techniques for signal transmission as opposed to narrowband channel techniques used in conventional analog systems. Accordingly, information contained in a transmission is spread over a very large bandwidth hence facilitating multiple users to share the same frequency band at the same time. This involves applying a code to data bits that specifically identify information belonging to a particular call (user) in a specific geographic location so that only the same code can read it at the receiver end. Thus, data bits for all users in a particular geographical cell, generally reachable by a local transceiver station, are simultaneously transmitted across the wide spectrum. A user's device picks up the signals and discards all the coded bits except for those specifically targeted to it. It then strips off the code and restores the transmission to its original data stream. Spread spectrum technique thus causes data to occupy more bandwidth than necessary by modulating a data signal with a spreading signal that uses more bandwidth than the data signal. Bits of spreading signal are called chips. Therefore, CDMA promotes privacy and security, as picking up a signal that is spread out over the spectrum requires knowledge of a code to separate one call from another as opposed to signals concentrated into narrow bands which are easier to monitor with radio receivers.
Each user in a CDMA communication system can be identified in various ways. Orthogonal multiple access is one such technique wherein a user is assigned one of the many orthogonal waveforms derived from an orthogonal code. Two signals are orthogonal if their cross-correlation is zero for a null time shift. Cross correlation is a general measure of the extent to which the two different signals, each shifted in time with respect to the other, correlate as a function of that time shift. Therefore, the orthogonal signals/waveforms assigned to different users transmitted at the same time will not interfere with each other.
An example for a set of orthogonal code is the Walsh set. Walsh functions are generated using an iterative process of constructing a Hadamard matrix starting with H1=[0]. Walsh functions are thus used for two purposes in CDMA. In a base station-to-mobile terminal forward link, they are utilized for creating independent transmission channels. In the reverse link from a mobile terminal to the base station, they are used for orthogonal modulation. However, a message signal processed via orthogonal Walsh sequences, is susceptible to false alarms wherein a time-shifted version of a transmitted Walsh sequence is erroneously identified as the transmitted data as well as channel decorrelation leading to performance degradation.